High performance coating blades are often used for applying a thin layer of coating color onto a traveling paper web. The influence of the paper fibers together with the high mineral content of pigments in the coating color and the high speed of modern blade coating installations, result in a situation where the blade tip is subjected to intense wear during use.
One of the first documents describing the use of ceramic tipped blades in order to increase the working life of coating blades, and thereby improve productivity in the coating process, is GB 2 130 924.
Document WO 98/26877 describes the use of a blade provided with a soft elastomer tip in order to provide a high performance coating blade having specific benefits relating to improvement of fiber coverage.
Quite recently, another class of coating blades has been developed and introduced to the market. These are blades for which the wear resistant working edge comprises a metallic or carbide deposit (carbides with a metallic matrix acting as a binder), or a cermet deposit. Such blades have mainly been produced by thermal spraying, with subsequent grinding to obtain the desired geometrical edge properties. Such deposit offers a range of advantages in blade coating compared to the traditional blades comprising a ceramic deposit, oxide blends and the like. One advantage is that such blades provide a far superior wear resistance compared to ceramic tipped blades, with the benefit of increasing even further the productivity in the coating station. Further, a drawback of ceramic blades has always been the inherent brittleness, leading to possible flaws or chips at the working edge of the blades. Such flaws or chips may occur during manufacture of the blade, during handling of the blade, or even during use of the blade in coating operations. The result of chips or other flaws at the working edge may be linear defects in the coated product, called streaks, or may even lead to web breaks and loss of material. The high toughness of metal and carbide based materials leads to lower sensitivity to edge cracking and therefore provides important advantages both during manufacture and handling, as well as during use of the blade. Yet another advantage of blades of this kind compared to ceramic blades is that they are less susceptible to edge wear occurring at the coating color limit adjacent to the longitudinal edges of the paper web. In addition, metallic or carbide materials are well suited for deposition by HVOF (High Velocity Oxy Fuel) spraying. In HVOF, the material is sprayed onto a substrate at a higher kinetic energy compared to plasma spraying (this latter using higher thermal energy). Therefore, very dense deposits may be formed (having lower than 2% porosity), enhancing the mechanical properties and reducing the risk of foreign particles getting trapped in the porosities.
Thus, there are many advantages motivating the use of metallic, carbide or cermet based coating blades for improving the productivity in the paper mill and also for raising the quality of the produced product.